1. Field of the Invention
The present invention relates to a fuel cell membrane electrode assembly including an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode.
2. Description of the Related Art
In general, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. In the fuel cell, the solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). Each of the anode and the cathode includes electrode catalyst as an electrode catalyst layer and porous carbon as a gas diffusion layer. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a fuel cell. In use of the fuel cell, generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack, e.g., mounted in a vehicle.
In some cases, in the membrane electrode assembly, the surface size of one of electrodes (catalyst layer and diffusion layer) is smaller than the surface size of the solid polymer electrolyte membrane, and the surface size of the other electrode is the same as the surface size of the solid polymer electrolyte membrane, e.g., as a stepped-type MEA.
For example, in Japanese Laid-Open Patent Publication No. 2009-099265 (hereinafter referred to as conventional technique), as shown in FIG. 16, an anode 2 and a cathode 3 are provided on both surfaces of a solid polymer electrolyte membrane 1. Gas seals 4a, 4b are provided around the anode 2 and the cathode 3. The anode 2 and the cathode 3 are sandwiched between separators 5, 6.
The anode 2 includes an anode catalyst layer 2a provided on one surface of the solid polymer electrolyte membrane 1 and an electrode base member (diffusion layer) 2b provided outside the anode catalyst layer 2a. The cathode 3 includes a cathode catalyst layer 3a provided on the other surface of the solid polymer electrolyte membrane 1 and an electrode base member (diffusion layer) 3b provided outside the cathode catalyst layer 3a. 
The outer shape of the anode catalyst layer 2a and the outer shape of the electrode base member 2b have the same size, and the outer shape of the cathode catalyst layer 3a and the outer shape of the electrode base member 3b have the same shape. Further, the outer shape of the anode 2 is larger than the outer shape of the cathode 3.
A fuel gas flow field 5a for supplying a fuel gas to the anode 2 is formed in the separator 5, and an oxygen-containing gas flow field 6a for supplying an oxygen-containing gas to the cathode 3 is formed in the separator 6.